Paweontowogy or pawaeontowogy (/ˌpeɪwiɒnˈtɒwədʒi, ˌpæwi-, -ən-/) is de scientific study of wife dat existed prior to, and sometimes incwuding, de start of de Howocene Epoch (roughwy 11,700 years before present). It incwudes de study of fossiws to determine organisms' evowution and interactions wif each oder and deir environments (deir paweoecowogy). Paweontowogicaw observations have been documented as far back as de 5f century BC. The science became estabwished in de 18f century as a resuwt of Georges Cuvier's work on comparative anatomy, and devewoped rapidwy in de 19f century. The term itsewf originates from Greek παλαιός, pawaios, "owd, ancient", ὄν, on (gen, uh-hah-hah-hah.ontos), "being, creature" and λόγος, wogos, "speech, dought, study".[1]

Body fossiws and trace fossiws are de principaw types of evidence about ancient wife, and geochemicaw evidence has hewped to decipher de evowution of wife before dere were organisms warge enough to weave body fossiws. Estimating de dates of dese remains is essentiaw but difficuwt: sometimes adjacent rock wayers awwow radiometric dating, which provides absowute dates dat are accurate to widin 0.5%, but more often paweontowogists have to rewy on rewative dating by sowving de "jigsaw puzzwes" of biostratigraphy. Cwassifying ancient organisms is awso difficuwt, as many do not fit weww into de Linnaean taxonomy dat is commonwy used for cwassifying wiving organisms, and paweontowogists more often use cwadistics to draw up evowutionary "famiwy trees". The finaw qwarter of de 20f century saw de devewopment of mowecuwar phywogenetics, which investigates how cwosewy organisms are rewated by measuring how simiwar de DNA is in deir genomes. Mowecuwar phywogenetics has awso been used to estimate de dates when species diverged, but dere is controversy about de rewiabiwity of de mowecuwar cwock on which such estimates depend.

The simpwest definition is "de study of ancient wife".[2] Paweontowogy seeks information about severaw aspects of past organisms: "deir identity and origin, deir environment and evowution, and what dey can teww us about de Earf's organic and inorganic past".[3]

Paweontowogy is one of de historicaw sciences, awong wif archaeowogy, geowogy, astronomy, cosmowogy, phiwowogy and history itsewf.[4] This means dat it aims to describe phenomena of de past and reconstruct deir causes.[5] Hence it has dree main ewements: description of de phenomena; devewoping a generaw deory about de causes of various types of change; and appwying dose deories to specific facts.[4]
When trying to expwain past phenomena, paweontowogists and oder historicaw scientists often construct a set of hypodeses about de causes and den wook for a smoking gun, a piece of evidence dat indicates dat one hypodesis is a better expwanation dan oders. Sometimes de smoking gun is discovered by a fortunate accident during oder research. For exampwe, de discovery by Luis Awvarez and Wawter Awvarez of an iridium-rich wayer at de Cretaceous–Tertiary boundary made asteroid impact and vowcanism de most favored expwanations for de Cretaceous–Paweogene extinction event.[5]

The oder main type of science is experimentaw science, which is often said to work by conducting experiments to disprove hypodeses about de workings and causes of naturaw phenomena – note dat dis approach cannot confirm a hypodesis is correct, since some water experiment may disprove it. However, when confronted wif totawwy unexpected phenomena, such as de first evidence for invisibwe radiation, experimentaw scientists often use de same approach as historicaw scientists: construct a set of hypodeses about de causes and den wook for a "smoking gun".[5]

Paweontowogy wies on de boundary between biowogy and geowogy since paweontowogy focuses on de record of past wife but its main source of evidence is fossiws,[6] which are found in rocks.[7] For historicaw reasons paweontowogy is part of de geowogy departments of many universities, because in de 19f century and earwy 20f century geowogy departments found paweontowogicaw evidence important for estimating de ages of rocks whiwe biowogy departments showed wittwe interest.[8]

Paweontowogy awso has some overwap wif archaeowogy, which primariwy works wif objects made by humans and wif human remains, whiwe paweontowogists are interested in de characteristics and evowution of humans as organisms. When deawing wif evidence about humans, archaeowogists and paweontowogists may work togeder – for exampwe paweontowogists might identify animaw or pwant fossiws around an archaeowogicaw site, to discover what de peopwe who wived dere ate; or dey might anawyze de cwimate at de time when de site was inhabited by humans.[9]

In addition paweontowogy often uses techniqwes derived from oder sciences, incwuding biowogy, osteowogy, ecowogy, chemistry, physics and madematics.[2] For exampwe, geochemicaw signatures from rocks may hewp to discover when wife first arose on Earf,[10] and anawyses of carbonisotope ratios may hewp to identify cwimate changes and even to expwain major transitions such as de Permian–Triassic extinction event.[11] A rewativewy recent discipwine, mowecuwar phywogenetics, often hewps by using comparisons of different modern organisms' DNA and RNA to re-construct evowutionary "famiwy trees"; it has awso been used to estimate de dates of important evowutionary devewopments, awdough dis approach is controversiaw because of doubts about de rewiabiwity of de "mowecuwar cwock".[12] Techniqwes devewoped in engineering have been used to anawyse how ancient organisms might have worked, for exampwe how fast Tyrannosaurus couwd move and how powerfuw its bite was.[13][14] It is rewativewy commonpwace to study fossiws using X-ray microtomography[15] A combination of paweontowogy, biowogy, and archaeowogy, paweoneurobiowogy is de study of endocraniaw casts (or endocasts) of species rewated to humans to wearn about de evowution of human brains.[16]

Paweontowogy even contributes to astrobiowogy, de investigation of possibwe wife on oder pwanets, by devewoping modews of how wife may have arisen and by providing techniqwes for detecting evidence of wife.[17]

Biostratigraphy, de use of fossiws to work out de chronowogicaw order in which rocks were formed, is usefuw to bof paweontowogists and geowogists.[25]Biogeography studies de spatiaw distribution of organisms, and is awso winked to geowogy, which expwains how Earf's geography has changed over time.[26]

Fossiws of organisms' bodies are usuawwy de most informative type of evidence. The most common types are wood, bones, and shewws.[27] Fossiwisation is a rare event, and most fossiws are destroyed by erosion or metamorphism before dey can be observed. Hence de fossiw record is very incompwete, increasingwy so furder back in time. Despite dis, it is often adeqwate to iwwustrate de broader patterns of wife's history.[28] There are awso biases in de fossiw record: different environments are more favorabwe to de preservation of different types of organism or parts of organisms.[29] Furder, onwy de parts of organisms dat were awready minerawised are usuawwy preserved, such as de shewws of mowwuscs. Since most animaw species are soft-bodied, dey decay before dey can become fossiwised. As a resuwt, awdough dere are 30-pwus phywa of wiving animaws, two-dirds have never been found as fossiws.[30]

Occasionawwy, unusuaw environments may preserve soft tissues. These wagerstätten awwow paweontowogists to examine de internaw anatomy of animaws dat in oder sediments are represented onwy by shewws, spines, cwaws, etc. – if dey are preserved at aww. However, even wagerstätten present an incompwete picture of wife at de time. The majority of organisms wiving at de time are probabwy not represented because wagerstätten are restricted to a narrow range of environments, e.g. where soft-bodied organisms can be preserved very qwickwy by events such as mudswides; and de exceptionaw events dat cause qwick buriaw make it difficuwt to study de normaw environments of de animaws.[31] The sparseness of de fossiw record means dat organisms are expected to exist wong before and after dey are found in de fossiw record – dis is known as de Signor–Lipps effect.[32]

Trace fossiws consist mainwy of tracks and burrows, but awso incwude coprowites (fossiw feces) and marks weft by feeding.[27][33] Trace fossiws are particuwarwy significant because dey represent a data source dat is not wimited to animaws wif easiwy fossiwised hard parts, and dey refwect organisms' behaviours. Awso many traces date from significantwy earwier dan de body fossiws of animaws dat are dought to have been capabwe of making dem.[34] Whiwst exact assignment of trace fossiws to deir makers is generawwy impossibwe, traces may for exampwe provide de earwiest physicaw evidence of de appearance of moderatewy compwex animaws (comparabwe to eardworms).[33]

Geochemicaw observations may hewp to deduce de gwobaw wevew of biowogicaw activity at a certain period, or de affinity of certain fossiws. For exampwe, geochemicaw features of rocks may reveaw when wife first arose on Earf,[10] and may provide evidence of de presence of eukaryotic cewws, de type from which aww muwticewwuwar organisms are buiwt.[35] Anawyses of carbonisotope ratios may hewp to expwain major transitions such as de Permian–Triassic extinction event.[11]

Naming groups of organisms in a way dat is cwear and widewy agreed is important, as some disputes in paweontowogy have been based just on misunderstandings over names.[37]Linnaean taxonomy is commonwy used for cwassifying wiving organisms, but runs into difficuwties when deawing wif newwy discovered organisms dat are significantwy different from known ones. For exampwe: it is hard to decide at what wevew to pwace a new higher-wevew grouping, e.g. genus or famiwy or order; dis is important since de Linnaean ruwes for naming groups are tied to deir wevews, and hence if a group is moved to a different wevew it must be renamed.[38]

Paweontowogists generawwy use approaches based on cwadistics, a techniqwe for working out de evowutionary "famiwy tree" of a set of organisms.[37] It works by de wogic dat, if groups B and C have more simiwarities to each oder dan eider has to group A, den B and C are more cwosewy rewated to each oder dan eider is to A. Characters dat are compared may be anatomicaw, such as de presence of a notochord, or mowecuwar, by comparing seqwences of DNA or proteins. The resuwt of a successfuw anawysis is a hierarchy of cwades – groups dat share a common ancestor. Ideawwy de "famiwy tree" has onwy two branches weading from each node ("junction"), but sometimes dere is too wittwe information to achieve dis and paweontowogists have to make do wif junctions dat have severaw branches. The cwadistic techniqwe is sometimes fawwibwe, as some features, such as wings or camera eyes, evowved more dan once, convergentwy – dis must be taken into account in anawyses.[36]

Paweontowogy seeks to map out how wiving dings have changed drough time. A substantiaw hurdwe to dis aim is de difficuwty of working out how owd fossiws are. Beds dat preserve fossiws typicawwy wack de radioactive ewements needed for radiometric dating. This techniqwe is our onwy means of giving rocks greater dan about 50 miwwion years owd an absowute age, and can be accurate to widin 0.5% or better.[41] Awdough radiometric dating reqwires very carefuw waboratory work, its basic principwe is simpwe: de rates at which various radioactive ewements decay are known, and so de ratio of de radioactive ewement to de ewement into which it decays shows how wong ago de radioactive ewement was incorporated into de rock. Radioactive ewements are common onwy in rocks wif a vowcanic origin, and so de onwy fossiw-bearing rocks dat can be dated radiometricawwy are a few vowcanic ash wayers.[41]

Conseqwentwy, paweontowogists must usuawwy rewy on stratigraphy to date fossiws. Stratigraphy is de science of deciphering de "wayer-cake" dat is de sedimentary record, and has been compared to a jigsaw puzzwe.[42] Rocks normawwy form rewativewy horizontaw wayers, wif each wayer younger dan de one underneaf it. If a fossiw is found between two wayers whose ages are known, de fossiw's age must wie between de two known ages.[43] Because rock seqwences are not continuous, but may be broken up by fauwts or periods of erosion, it is very difficuwt to match up rock beds dat are not directwy next to one anoder. However, fossiws of species dat survived for a rewativewy short time can be used to wink up isowated rocks: dis techniqwe is cawwed biostratigraphy. For instance, de conodont Eopwacognadus pseudopwanus has a short range in de Middwe Ordovician period.[44] If rocks of unknown age are found to have traces of E. pseudopwanus, dey must have a mid-Ordovician age. Such index fossiws must be distinctive, be gwobawwy distributed and have a short time range to be usefuw. However, misweading resuwts are produced if de index fossiws turn out to have wonger fossiw ranges dan first dought.[45] Stratigraphy and biostratigraphy can in generaw provide onwy rewative dating (A was before B), which is often sufficient for studying evowution, uh-hah-hah-hah. However, dis is difficuwt for some time periods, because of de probwems invowved in matching up rocks of de same age across different continents.[46]

Famiwy-tree rewationships may awso hewp to narrow down de date when wineages first appeared. For instance, if fossiws of B or C date to X miwwion years ago and de cawcuwated "famiwy tree" says A was an ancestor of B and C, den A must have evowved more dan X miwwion years ago.

It is awso possibwe to estimate how wong ago two wiving cwades diverged – i.e. approximatewy how wong ago deir wast common ancestor must have wived – by assuming dat DNA mutations accumuwate at a constant rate. These "mowecuwar cwocks", however, are fawwibwe, and provide onwy a very approximate timing: for exampwe, dey are not sufficientwy precise and rewiabwe for estimating when de groups dat feature in de Cambrian expwosion first evowved,[47] and estimates produced by different techniqwes may vary by a factor of two.[12]

The evowutionary history of wife stretches back to over 3,000 miwwion years ago, possibwy as far as 3,800 miwwion years ago.[49]Earf formed about 4,570 miwwion years ago and, after a cowwision dat formed de Moon about 40 miwwion years water, may have coowed qwickwy enough to have oceans and an atmosphere about 4,440 miwwion years ago.[50] However, dere is evidence on de Moon of a Late Heavy Bombardment from 4,000 to 3,800miwwion years ago. If, as seems wikewy, such a bombardment struck Earf at de same time, de first atmosphere and oceans may have been stripped away.[51] The owdest cwear evidence of wife on Earf dates to 3,000 miwwion years ago, awdough dere have been reports, often disputed, of fossiwbacteria from 3,400 miwwion years ago and of geochemicaw evidence for de presence of wife 3,800 miwwion years ago.[10][52] Some scientists have proposed dat wife on Earf was "seeded" from ewsewhere,[53] but most research concentrates on various expwanations of how wife couwd have arisen independentwy on Earf.[54]

For about 2,000 miwwion years microbiaw mats, muwti-wayered cowonies of different types of bacteria, were de dominant wife on Earf.[55] The evowution of oxygenic photosyndesis enabwed dem to pway de major rowe in de oxygenation of de atmosphere[56] from about 2,400 miwwion years ago. This change in de atmosphere increased deir effectiveness as nurseries of evowution, uh-hah-hah-hah.[57] Whiwe eukaryotes, cewws wif compwex internaw structures, may have been present earwier, deir evowution speeded up when dey acqwired de abiwity to transform oxygen from a poison to a powerfuw source of energy in deir metabowism. This innovation may have come from primitive eukaryotes capturing oxygen-powered bacteria as endosymbionts and transforming dem into organewwes cawwed mitochondria.[49][58] The earwiest evidence of compwex eukaryotes wif organewwes such as mitochondria, dates from 1,850 miwwion years ago.[22]

Muwticewwuwar wife is composed onwy of eukaryotic cewws, and de earwiest evidence for it is de Franceviwwian Group Fossiws from 2,100 miwwion years ago,[59] awdough speciawisation of cewws for different functions first appears between 1,430 miwwion years ago (a possibwe fungus) and 1,200 miwwion years ago (a probabwe red awga). Sexuaw reproduction may be a prereqwisite for speciawisation of cewws, as an asexuaw muwticewwuwar organism might be at risk of being taken over by rogue cewws dat retain de abiwity to reproduce.[60][61]

The earwiest known animaws are cnidarians from about 580 miwwion years ago, but dese are so modern-wooking dat de earwiest animaws must have appeared before den, uh-hah-hah-hah.[62] Earwy fossiws of animaws are rare because dey did not devewop minerawised hard parts dat fossiwise easiwy untiw about 548 miwwion years ago.[63] The earwiest modern-wooking biwaterian animaws appear in de Earwy Cambrian, awong wif severaw "weird wonders" dat bear wittwe obvious resembwance to any modern animaws. There is a wong-running debate about wheder dis Cambrian expwosion was truwy a very rapid period of evowutionary experimentation; awternative views are dat modern-wooking animaws began evowving earwier but fossiws of deir precursors have not yet been found, or dat de "weird wonders" are evowutionary "aunts" and "cousins" of modern groups.[64]Vertebrates remained an obscure group untiw de first fish wif jaws appeared in de Late Ordovician.[65][66]

At about 13 centimetres (5.1 in) de Earwy Cretaceous Yanoconodon was wonger dan de average mammaw of de time.[67]

The spread of wife from water to wand reqwired organisms to sowve severaw probwems, incwuding protection against drying out and supporting demsewves against gravity.[68][69][70][71] The earwiest evidence of wand pwants and wand invertebrates date back to about 476 miwwion years ago and 490 miwwion years ago respectivewy.[70][72] The wineage dat produced wand vertebrates evowved water but very rapidwy between 370 miwwion years ago and 360 miwwion years ago;[73] recent discoveries have overturned earwier ideas about de history and driving forces behind deir evowution, uh-hah-hah-hah.[74] Land pwants were so successfuw dat dey caused an ecowogicaw crisis in de Late Devonian, untiw de evowution and spread of fungi dat couwd digest dead wood.[24]

During de Permian period synapsids, incwuding de ancestors of mammaws, may have dominated wand environments,[76] but de Permian–Triassic extinction event251 miwwion years ago came very cwose to wiping out compwex wife.[77] The extinctions were apparentwy fairwy sudden, at weast among vertebrates.[78] During de swow recovery from dis catastrophe a previouswy obscure group, archosaurs, became de most abundant and diverse terrestriaw vertebrates. One archosaur group, de dinosaurs, were de dominant wand vertebrates for de rest of de Mesozoic,[79] and birds evowved from one group of dinosaurs.[75] During dis time mammaws' ancestors survived onwy as smaww, mainwy nocturnaw insectivores, but dis apparent set-back may have accewerated de devewopment of mammawian traits such as endodermy and hair.[80] After de Cretaceous–Paweogene extinction event66 miwwion years ago[81] kiwwed off de non-avian dinosaurs – birds are de onwy surviving dinosaurs – mammaws increased rapidwy in size and diversity, and some took to de air and de sea.[82][83][84]

Fossiw evidence indicates dat fwowering pwants appeared and rapidwy diversified in de Earwy Cretaceous, between 130 miwwion years ago and 90 miwwion years ago.[85] Their rapid rise to dominance of terrestriaw ecosystems is dought to have been propewwed by coevowution wif powwinating insects.[86]Sociaw insects appeared around de same time and, awdough dey account for onwy smaww parts of de insect "famiwy tree", now form over 50% of de totaw mass of aww insects.[87]

Humans evowved from a wineage of upright-wawking apes whose earwiest fossiws date from over 6 miwwion years ago.[88] Awdough earwy members of dis wineage had chimp-sized brains, about 25% as big as modern humans', dere are signs of a steady increase in brain size after about 3 miwwion years ago.[89] There is a wong-running debate about wheder modern humans are descendants of a singwe smaww popuwation in Africa, which den migrated aww over de worwd wess dan 200,000 years ago and repwaced previous hominine species, or arose worwdwide at de same time as a resuwt of interbreeding.[90]

Life on earf has suffered occasionaw mass extinctions at weast since 542 miwwion years ago. Awdough dey are disasters at de time, mass extinctions have sometimes accewerated de evowution of wife on earf. When dominance of particuwar ecowogicaw niches passes from one group of organisms to anoder, it is rarewy because de new dominant group is "superior" to de owd and usuawwy because an extinction event ewiminates de owd dominant group and makes way for de new one.[91][92]

The fossiw record appears to show dat de rate of extinction is swowing down, wif bof de gaps between mass extinctions becoming wonger and de average and background rates of extinction decreasing. However, it is not certain wheder de actuaw rate of extinction has awtered, since bof of dese observations couwd be expwained in severaw ways:[93]

The oceans may have become more hospitabwe to wife over de wast 500 miwwion years and wess vuwnerabwe to mass extinctions: dissowved oxygen became more widespread and penetrated to greater depds; de devewopment of wife on wand reduced de run-off of nutrients and hence de risk of eutrophication and anoxic events; marine ecosystems became more diversified so dat food chains were wess wikewy to be disrupted.[94][95]

Reasonabwy compwete fossiws are very rare, most extinct organisms are represented onwy by partiaw fossiws, and compwete fossiws are rarest in de owdest rocks. So paweontowogists have mistakenwy assigned parts of de same organism to different genera, which were often defined sowewy to accommodate dese finds – de story of Anomawocaris is an exampwe of dis.[96] The risk of dis mistake is higher for owder fossiws because dese are often unwike parts of any wiving organism. Many "superfwuous" genera are represented by fragments dat are not found again, and dese "superfwuous" genera appear to become extinct very qwickwy.[93]

Awdough paweontowogy became estabwished around 1800, earwier dinkers had noticed aspects of de fossiw record. The ancient Greek phiwosopherXenophanes (570–480 BC) concwuded from fossiw sea shewws dat some areas of wand were once under water.[98] During de Middwe Ages de Persian naturawist Ibn Sina, known as Avicenna in Europe, discussed fossiws and proposed a deory of petrifying fwuids on which Awbert of Saxony ewaborated in de 14f century.[99] The Chinese naturawist Shen Kuo (1031–1095) proposed a deory of cwimate change based on de presence of petrifiedbamboo in regions dat in his time were too dry for bamboo.[100]

In earwy modern Europe, de systematic study of fossiws emerged as an integraw part of de changes in naturaw phiwosophy dat occurred during de Age of Reason. In de Itawian Renaissance, Leonardo Da Vinci made various significant contributions to de fiewd as weww depicted numerous fossiws. Leonardo's contributions are centraw to de history of paweontowogy because he estabwished a wine of continuity between de two main branches of paweontowogy—ichnowogy and body fossiw paweontowogy[101][102][103]. He identified de fowwowing[101]:

The biogenic nature of ichnofossiws, i.e. ichnofossiws were structures weft by wiving organisms;

The utiwity of ichnofossiws as paweoenvironmentaw toows—certain ichnofossiws show de marine origin of rock strata;

The importance of de neoichnowogicaw approach—recent traces are a key to understanding ichnofossiws;

The independence and compwementary evidence of ichnofossiws and body fossiws—ichnofossiws are distinct from body fossiws, but can be integrated wif body fossiws to provide paweontowogicaw information

At de end of de 18f century Georges Cuvier's work estabwished comparative anatomy as a scientific discipwine and, by proving dat some fossiw animaws resembwed no wiving ones, demonstrated dat animaws couwd become extinct, weading to de emergence of paweontowogy.[104] The expanding knowwedge of de fossiw record awso pwayed an increasing rowe in de devewopment of geowogy, particuwarwy stratigraphy.[105]

The first hawf of de 19f century saw geowogicaw and paweontowogicaw activity become increasingwy weww organised wif de growf of geowogic societies and museums[106][107] and an increasing number of professionaw geowogists and fossiw speciawists. Interest increased for reasons dat were not purewy scientific, as geowogy and paweontowogy hewped industriawists to find and expwoit naturaw resources such as coaw.[108]

This contributed to a rapid increase in knowwedge about de history of wife on Earf and to progress in de definition of de geowogic time scawe, wargewy based on fossiw evidence. In 1822 Henri Marie Ducrotay de Bwanviwwe, editor of Journaw de Physiqwe, coined de word "pawaeontowogy" to refer to de study of ancient wiving organisms drough fossiws.[109] As knowwedge of wife's history continued to improve, it became increasingwy obvious dat dere had been some kind of successive order to de devewopment of wife. This encouraged earwy evowutionary deories on de transmutation of species.[110]
After Charwes Darwin pubwished Origin of Species in 1859, much of de focus of paweontowogy shifted to understanding evowutionary pads, incwuding human evowution, and evowutionary deory.[110]

The wast hawf of de 19f century saw a tremendous expansion in paweontowogicaw activity, especiawwy in Norf America.[112] The trend continued in de 20f century wif additionaw regions of de Earf being opened to systematic fossiw cowwection, uh-hah-hah-hah. Fossiws found in China near de end of de 20f century have been particuwarwy important as dey have provided new information about de earwiest evowution of animaws, earwy fish, dinosaurs and de evowution of birds.[113] The wast few decades of de 20f century saw a renewed interest in mass extinctions and deir rowe in de evowution of wife on Earf.[114] There was awso a renewed interest in de Cambrian expwosion dat apparentwy saw de devewopment of de body pwans of most animaw phywa. The discovery of fossiws of de Ediacaran biota and devewopments in paweobiowogy extended knowwedge about de history of wife back far before de Cambrian, uh-hah-hah-hah.[64]